For a rather long period, it was assumed that gram-negative bacteria do not "secrete" proteins into their environment but only export proteins in their strategic periplasm. However, research in the last two decades has revealed that gram-negative bacteria do indeed transfer proteins across their sophisticated outer membrane, and they do this by a variety of systems that are now classified into four major types and several minor ones. Type I, exemplified by the hemolysin secretion system of Escherichia coli, is a rather simple exporter that is based on only three proteins, one of which belongs to the ABC transporters. Type II is a very complex apparatus that extends the general secretory pathway and transfers fully folded enzymes or toxins from the periplasm to the extracellular medium, across the outer membrane. Type IV, another complex system that transfers pertussis toxin among others, is related to the apparatus of Agrobacterium spp. that transfers DNA to plant cells. Finally, type III, the subject of this review, is a sophisticated apparatus that couples secretion with pathogenesis.

In bacteria that are pathogenic for animals, type III secretion systems allow extracellular bacteria adhering to the surface of a host cell to inject specialized proteins across the plasma membrane. This system probably also allows bacteria residing in vacuoles to inject proteins across the vacuolar membrane. The injected proteins subvert the functioning of the aggressed cell or destroy its communications, favoring the entry or survival of the invading bacteria. Type III is thus not a secretion apparatus in the strict sense of the term but rather a complex weapon for close combat. It contributes to a number of totally different animal diseases with a variety of symptoms and severities, from fatal septicemia to mild diarrhea and from fulgurant diarrhea to chronic infection of the lung. Type III secretion has been extensively studied in Yersinia spp. (reviewed in 25), in Salmonella spp. (reviewed in 47), in Shigella spp. (reviewed in 138), and in enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) (40, 50, 72). It has also been described in Pseudomonas aeruginosa (TL Yahr & DW Frank, Genbank PAU56077), Chlamydia trachomatis and Chlamydia pneumoniae (73A), Bordetella bronchiseptica (MH Yuk, ET Harvill, JF Miller, Genbank AFO49488), Bordetella pertussis (78A) and in Burkholderia pseudomallei (The Sanger Center, Cambridge, UK). It is surprising that Salmonella typhimurium and Yersinia spp. have not only one type III system but two (61, 104; S Carlson & DE Pierson, Genbank AFO055744; The Sanger Center, Cambridge, UK), presumably playing their role at different stages of the infection (Figure 1).

Type III systems in animal pathogens. Illustrated are the various bacterial pathogens endowed with type III secretion, injecting effectors into the cytosol of a eukaryotic target cell. See Table 3 for references.

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(2) In the "Flagella aren't necessarily good for you either" category: